1. Field of the Invention
The present invention relates to an elastic boundary wave device for use in a resonator, a band-pass filter, or other suitable device, and more particularly, the present invention relates to an elastic boundary wave device including an IDT disposed between a first medium and a second medium, which are made of different materials.
2. Description of the Related Art
An elastic boundary wave device includes interdigital electrodes (IDTs) that are disposed on an interface between different media. An elastic boundary wave propagates through a layered body of the different media in the elastic boundary wave device. Thus, the elastic boundary wave device does not require a complicated package structure, and has a simpler, lower-profile structure than surface acoustic wave devices.
An elastic boundary wave device that operates at a higher frequency includes IDTs that have a shorter period. This decreases the width of electrode fingers of an IDT or a reflector, and thus, increases the conductor resistance and the loss of the electrode fingers.
In an elastic boundary wave device, when the sonic velocity of an elastic boundary wave is less than the sonic velocity of a transverse wave propagating through the media disposed over and under the interface, the elastic boundary wave is confined or trapped between the upper medium and the lower medium. This reduces the propagation loss.
To enhance such a confinement effect, it is effective to provide an IDT made of a high-density metal. As described in Japanese Unexamined Patent Application Publication No. 58-30217 (Patent Document 1), many elastic boundary wave devices include IDTs made of Al. In contrast, in an elastic boundary wave device described in DE 4132309 A1 (Patent Document 2), in addition to Al, Au and Ag are described as materials used for IDTs.
In an elastic boundary wave device, the operating frequency Fi at which an IDT generates an elastic boundary wave is expressed by Fi=V/λi (equation (a)), where V denotes the sonic velocity of an elastic boundary wave and λi denotes the period of IDT. As is apparent from the equation (a), when the operating frequency Fi of an elastic boundary wave device is increased, the period λi of the IDT must be decreased. This also decreases the width of electrode fingers of an IDT or a reflector, thus, increasing the conductor resistance and the loss. In particular, when electrode fingers are made of a high-density conductor, the propagation loss is close to zero if the thickness of the electrode fingers is small. However, as the thickness of the electrode fingers decreases, the conductor resistance increases.
Previously, when IDTs of an elastic boundary wave device were made of Au, the electromechanical coupling coefficient K2 was not sufficiently large, and the temperature coefficient of delay (TCD) was not sufficiently small.